In certain machining operations where the cutting is performed by grit or teeth carried on a surface of highly precise flatness, it is desirable to regulate the final profile of the machined surface, so as to locate all of a series of features carried on a surface intersecting the one machined to within a preselected tolerance band from the machined surface. In one such typical operation, 13 features are located on 0.16 in. centers and ideally the surface must be machined so that each feature is finally located from 20 to 80 uin. from the surface. It is desirable to produce this final configuration regardless of the spacing of these features from either the original surface to be lapped or otherwise machined or from another datum line.
The typical operation mentioned above arises in the batch fabrication of thin film magnetic heads for disk memories. The features comprise individual thin film head throats deposited on the workpiece's side and so located, relative to the original position of the surface to be machined, as to require substantial machining of the surface. After the machining process is complete, the bar is sliced transversely to free the individual transducer heads.
These heads can not be deposited precisely along any datum line due to errors in the mask controlling the deposition, errors in position of the mask, and changes in bar geometry caused by external force and residual stress. Nonetheless, all of the head throats must be located within the predetermined 20-80 uin. tolerance band from the edge in order for them to be acceptable. Obviously, the fewer reject heads resulting from a given workpiece, the more efficient and economical the operation is. Heretofore it has not been possible to locate high percentages of the features simultaneously within the specified tolerance band at any stage of the machining process.
It is also possible to place sensors at predetermined positions on the bar to be machined, to determine the progress of the machining operation. In fact, the sensors which we are using provide a continuous and highly accurate indication (to 5 uin. or better) of the spacing of the surface being machined from each feature. Therefore, it is possible to monitor during the machining process the progress of each of the features toward the preselected tolerance band.